Precision timing is required between the needle and bobbin movements of a sewing machine. In order to link the motion of the needle to that of the bobbin, conventional sewing machines use mechanical linkages, gears, drive shafts, timing belt, and other mechanisms to mechanically connect and mechanically gear the bobbin to the needle. An arm is used to position the needle above the bobbin, with the linkages between the needle and bobbin being routed through the arm. A single motor is typically used to drive both needle and bobbin by way of the mechanical gearing connecting them to each other.
Several disadvantages result from mechanically linking the needle and bobbin. The speed of the sewing machine is limited by the inertia and friction caused by the mechanical linkages. These mechanical linkages require constant lubrication. Reliability suffers from a system that employs so many moving parts. Additional power is required to accelerate and decelerate the mass of linkages. As a result, greater heat dissipation is required to prevent overheating. Noise levels are increased as a result of these moving mechanical parts. Finally, the ergonomic disadvantages associated with mechanically linking the needle and bobbin diminishes the sewing machine's versatility by limiting the mobility and location of the sewing head (needle and bobbin).
Attempts have been made to physically separate the needle and bobbin and to use separate electric motors to synchronize the needle with the bobbin. In U.S. Pat. No. 3,515,080, Ramsey discloses a sewing machine having physically separate needle and bobbin drive units that are purportedly synchronized. Stepper motors are disclosed as the drive units, and each drive unit is "electrically connected and operated in synchronism and in unison". However, Ramsey does not disclose a system or mechanism for electronically linking (gearing) the needle to the bobbin. Ramsey discloses in some detail a control system for moving the needle and bobbin laterally in the X-Y plane and for rotating them about the Z-axis, but there is little or no disclosure of how the needle and bobbin are driven or controlled for sewing (i.e., how the up and down motion of the needle and the rotation of the bobbin is controlled so that the bobbin hook engages and hooks the thread carried by the needle).
The prior art provides an inflexible approach to coordinating the needle and bobbin movements during sewing. Whenever a needle type is changed, or the material thickness varies, or the bobbin is switched to a looper, a sewing parameter has been varied. Prior art machines are unable to automatically adjust to these changed parameters. Instead, prior art machines employing physically separate needle and bobbin must be manually reset to function within the new parameters.